Too bad it's still the 2nd of April, not the 23rd, when I could say "the Ionmonkey tree has had exactly one push in the last year." Still, it' been nearly a year since it was used, a year during which Try became a perfectly useful place to push spidermonkey experiments, since it now does all the shell builds and hazard builds you could want (and I don't really know whether or not Ionmonkey actually does a current full set).
Last week, we switched the default for nightlies from building on the last good rev, which also switched them from looking at whether or not a nightly had been built on the last rev, to just building on the tip.
Since then, we've built six pointless nightlies on the Ionmonkey tree, and triggered six full sets of pointless mostly-unrunnable tests on it. The fact that we're out $60 for that is minimal, compared to the fact that we're out the use of those slaves during a critical time of the day.
Best choice: decide that Ionmonkey has done its job, and that now Try and the occasional use of a twig is a better use of our resources, and retire it.
Okay choice: decide that what's really a barely-used twig doesn't actually need nightlies, and shut them off.
Really not so great a choice: set enable_nightly_lastgood to True on it, so that it won't build another nightly on a rev that has already had one (and also won't build a nightly at all if any platform is permared, or just doesn't happen to successfully build on the tip push).
Your call, bhackett, reassign to nobody@ once you've picked one.

(In reply to Phil Ringnalda (:philor) from comment #0)
> Last week, we switched the default for nightlies from building on the last
> good rev, which also switched them from looking at whether or not a nightly
> had been built on the last rev, to just building on the tip.
(I was about to file a bug for this but then found bug 968075, for anyone else who was thinking of doing the same.)